DEVELOPMENT OF BROAD RANGE SCAN CAPABILITIES WITH JET COOLED CAVITY RINGDOWN SPECTROSCOPY
Terrance Codd, Ming-Wei Chen, Terry A. Miller
The Ohio State University
Cavity Ringdown Spectroscopy
A =σNl
Photo-diode
τ0
cL )/(R1 -
= τabs
σ Nl+= cL )/(
R1-( ) tabs
Time
Inte
nsi
ty
0
A = L/(cτabs)- L/(cτ0)
Sensitivity of TechniqueIf R = 99.99% and L = 67cmthen τ0 = 22.3 μsLeff = 6.7 km ∼ 4.16 Miles l = 5 cmleff = 0.5 km
l
L
Comparison of CRDS Methods for A-X NO3
HR JC-CRDS Specs
Radiation 100-250 MHz FWHM
Rotational temps 15-30 K Benefits
Resolved transitions can be used to accurately determine rotational and spin rotational constants and orientation of TDM
Drawbacks Radiation difficult to
produce and scans very time consuming
RT CRDS Specs
~2 GHz FWHM Deev et al, 4.5 GHz Ambient temperatures
Benefits Can quickly scan very
broad ranges and radiation is easy to generate
Drawbacks Higher rotational
temperature leads to congestion of transitions making assignment difficult.
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Deev, J. Sommar, M. Okumura J. Chem. Phys. 122, 2243051 (2005)
Jacox, M; Thompson, W. E.; J. Chem. Phys.; 122, 224305 (2005)
Combining Mod Res/Jet Cooling
Want to combine some of the benefits of moderate resolution radiation sources (broad range scan capabilities and ease of use) with the benefits of the jet cooled system (rotationally cold samples)
Goal: couple moderate resolution radiation with JC-CRDS
Mod-Res Jet Cooled CRDS
Fiber Optic works very well. Roughly 90% efficient transmission of IR
Nd:YAG
20 Hz
Sirah
Dye Laser
H2 Raman CellFilters
1st or 2nd Stokes2-10 mJ/
650 - 700 mJ 75 - 115 mJ
20 m Fiber Optic
Collimator
~2 GHz FWHM
N2O5 in First Run Neon
NO2 + NO3
PD
Previous CRDS of NO3
Previous CRDS spectra of A-X transition of NO3 have been done in the Okumura lab under ambient conditions. High rotational temps make band assignments difficult.
No good assignment of the 3 band
401
201
30140
2
10140
1
10120
1
403
10140
2
A. Deev, J. Sommar, M. Okumura J. Chem. Phys. 122, 2243051 (2005)
~ ~
Comparison To Room Temp
7500 8000 8500-100000
-80000
-60000
-40000
-20000
0
20000
40000
60000
80000
100000
120000
a.u
.
wavenumber
8500 9000 9500-100000
-80000
-60000
-40000
-20000
0
20000
40000
60000
80000
100000
120000
a.u
.
wavenumber
Ambient NO3 data courtesy of M. Okumura
Comparison To Room Temp
9500 10000 10500-100000
-80000
-60000
-40000
-20000
0
20000
40000
60000
80000
100000
120000
a.u
.
wavenumber
Ambient NO3 data courtesy of M. Okumura
NO3 Radical 401 Band
See intensity cut by ~2/3 compared to HR Can resolve rotational structure but not individual
transitions.
7590 7600 7610 7620
-3
-2
-1
0
1
2
3
4
ModRes (30 K) HiRes*0.3 -3ppm (15 K)
pp
m
wavenumber
TD07
Characterizing Method
Have lower signal to noise ratio than with high-resolution radiation
Reduction in signal probably due to the fact that the transitions being observed have a much narrower linewidth than the radiation Signal intensity is limited by both intensity and density
of transitions Increase in noise probably caused by using radiation
which is much broader than FSR of cavity Means we span a number of longitudinal cavity modes Consistent with our experience on RT-CRDS
Provides sufficient sensitivity for weak transitions
7500 8000 8500 9000 9500 10000 10500
0
1
2
3
4
5
ppm
wavenumber
Making Assignments
We used these assignments as a starting point to get good fundamental frequencies for the vibrational modes.
401
201
30140
2
10140
1
10120
1
403
10140
2
A. Deev, J. Sommar, M. Okumura J. Chem. Phys. 122, 2243051 (2005)
401
201
402
10140
1
10120
140
3
10140
2
Predicting Transitions
To aid in the assignment of other transitions a set of predicted transitions were calculated.
Used pure harmonic approximations as a first guess based on fundamental frequency.
Made assignments based on that and then fit anharmonic constants and frequencies.
Done with all possible modes combinations NOT including any quanta of excitation in 3.
Predicted Assignments
7600 7800 8000 8200 8400
0
1
2
3
4
5
ppm
wavenumber
401 40
2 20140
1
10140
1
201 10
1 202
8600 8800 9000 9200 9400
0
1
2
3
4
5
ppm
wavenumber
Predicted Assignments
403 20
1402
20140
3
10140
2
10140
320
3
10140
1 20240
1
10120
1401
10240
1
102
10120
2 103
10220
1
404
Predicted Assignments
9600 9800 10000 10200 10400
0
1
2
3
4
5
ppm
wavenumber
20340
1
10240
2 20140
410
2403
20340
2
3 Assignment
Most prominent unassigned transition in the lower frequency range is the weak parallel band at 8753 cm-1. This is 1689 cm-1
from the origin. Eisfeld and Morokuma*1 have calculated the frequency of the 3
band to be 1602 cm-1. Previous assignment by Deev*2 for 3 was 300-400 cm-1 away from the predicted frequency and did not have parallel band contour.
Based on the contour and the predicted frequency we assign this to the 30
1 band.
1. W. Eisfeld, K. Morokuma, J. Chem. Phys. 114, 9430 (2001)
2. A. Deev, J. Sommar, M. Okumura J. Chem. Phys. 122, 2243051 (2005)
8600 8800 9000 9200 9400
0
1
2
3
4
5
ppm
wavenumber
403
20140
2
20140
3
10140
2
10140
320
3
10140
1 20240
1
10120
1401
10240
1
102 10
1202 10
3
10220
1404
Other Work
Wanted to use this apparatus to investigate other molecules as well
Hydroxy Propyl Peroxy radical is being studied on the RT-CRDS system
A jet cooled spectrum could aid in the assignment of some of the low frequency transitions observed
Conclusion
We have developed a moderate resolution jet cooled cavity ringdown spectrometer capable of quickly obtaining broad range scans with good sensitivity.
We have obtained spectra of the A-X transition of NO3 Almost all of the cold vibronic spectrum has been
assigned including the 301 band.
Additional broadband structure has been revealed which is coincident with vibronically forbidden transitions
Splitting in υ1υ4 combination bands have been observed
Ongoing analysis should provide more insight into the Jahn-Teller effect and vibronic coupling in the A state of NO3.
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